Rename _.indexBy to _.keyBy to avoid confusion with _.sortedIndexBy forms.

test/underscore.html

@@ -185,6 +185,8 @@
             'Lookup falsy iterator'
           ],
           'partition': [
+            'can reference the array index',
+            'Died on test #8',
             'partition takes a context argument',
             'function(a){[code]}'
           ],
@@ -317,6 +319,7 @@
           'extendOwn': 'assign',
           'findWhere': 'find',
           'include': 'includes',
+          'indexBy': 'keyBy',
           'inject': 'reduce',
           'mapObject': 'mapValues',
           'matcher': 'matches',